i Do have a hand with a gun(seperate meshes)! Sumtimes the hand gets invisble, if i move the mouse! And then visible back again!
How to fix that problem? (I can always see the gun! just the hand disappears sumtimes)
3D Software: Blender 2.49
If the two objects are two separate meshes then it could be a bounding box issue. Try moving your hand and gun to an unlikely position, like the middle of the screen, then see if it still disappears.
Maybe you somehow imported a strange/broken animation. Select the model in the Hierarchy and take a look at the Inspector. In the Animation section uncheck "Play automatically", this way there shouldn't be any animation active.
Another thing you could check is to start your game and wait for the hand to disappear, then pause your game and go into the Scene window. Take a look at the Hierarchy and open the tree of your model, select your hand mesh and mouseover to the Scene window, now press F. Unity should zoom to your hand mesh. Like this you can check if the hand is turning invisible or is just out of sight.